According to an X-ray CT apparatus, an X-ray tube for irradiating an examinee with X-ray and an X-ray detector for detecting the dose of X-ray transmitted through the examinee as projection data are rotated around the examinee, a tomographic image of the examinee is reconstructed by using projection data of plural angles detected by the X-ray detector and the reconstructed tomographic image is displayed and supplied for image diagnosis.
The X-ray tube has a cathode for discharging an electron beam and an anode for making the electron beam discharged from the cathode impinge thereto to generate X-ray. The electron beam is accelerated by a high voltage applied between the cathode and the anode, and converted to X-ray in the neighborhood of the surface in the anode when it impinges against the anode. A part of the energy of the accelerated electron beam is converted to X-ray, and most of the remaining part is converted to heat, so that the anode is heated and thermally expanded. As a result, the position on the surface of the anode which substantially serves as an occurrence point of X-ray varies, and thus the position of the X-ray beam varies. The variation of the X-ray occurrence point (also called as X-ray focal point), that is, the variation of the X-ray beam position causes a measurement error of projection data obtained by the X-ray detector and thus causes occurrence of an artifact (false image) on a tomographic image obtained after reconstruction. When an artifact occurs on a tomographic image, the diagnosis performance of the X-ray CT apparatus is remarkably deteriorated and thus it is necessary to correct and reduce the variation of the X-ray occurrence point.
With respect to the conventional correction of the variation amount of the X-ray beam, for example, Patent Document 1 is known. The Patent Document 1 stores the variation amount of X-ray beam due to an accumulated heat capacity in advance, grasps the state of the X-ray tube from the information on the information concerned at the actual measurement time and accumulated heat capacity information at that time point, and corrects the X-ray occurrence point, that is, the X-ray beam position of the X-ray tube.